Projective projectors, such as liquid crystal projectors and Digital Light Processing (trademark, DLP) projectors, which can display images on large-area screens, are effective for reproduction of real and powerful images. FIG. 7 shows a configuration of such a projector. The projector includes a lighting apparatus for a high-pressure discharge lamp provided with a high-pressure discharge lamp 10 having a reflective mirror and a power supply unit 30 supplying electric power to the lamp 10; a projector control unit 31; a light modulation device 32 composed of, for example, a liquid crystal panel; and a magnifying device 33 including a projection lens that projects enlarged images appearing in the light modulation device 34. Enlarged images expanded by the magnifying device 33 are projected onto a screen 32. The projector control unit 31 includes an image controller 31a that processes image signals received from an external unit 35 such as a personal computer or a television set and a lighting controller 31b that orders the power supply unit 30 to send a turning-on command and a turning power command for switching on the high-pressure discharge lamp.
In recent years, projectors have been further compacted, further simplified. and have been used in standard homes, in addition to commercial use. Such a trend requires a higher screen brightness so that the projectors can display any image in lighted environments and the type of images that and persuades projectors to have “light modulation functions” (for example, Japanese Patent Application Publication No. 2000-131668). The light modulation function involves lighting of the high-pressure discharge lamp at a low electric power that is much lower than its rated lighting power to control the brightness of the lamp and to reduce the power consumption. The electric power supplied by the light modulation function is about 25 to 80% of the rated power of the high-pressure discharge lamp.
Such a projector is generally provided with two operation modes, that is, “rated power lighting” and “economic power lighting,” involves an operation at a power that is about 80% of the rated power lighting. Since the “rated power lighting” and “economic power lighting” generally have very similar lighting waveforms, lighting of the lamp by the “rated power lighting” and “economic power lighting” is collectively referred to as “stationary power lighting” hereinafter, and its lighting mode is referred to as a “stationary power lighting mode.” Throughout the specification, the operation of the lamp using the light modulation function described above is referred to as “modulated power lighting” and its lighting mode is referred to as a “modulated lighting mode.”
It is preferred that the power of the lamp be reduced to the minimum in a modulated power lighting mode using such a light modulation function due to the following reasons: Lighting at low electric power reduces heat dissipation from the lamp. This can abolish or diminish the rotation of a cooling fan projector, which is the main cause of noise generation. The low-power lighting can also reduce thermal load to the lamp and thus can prolong the service life of the lamp. The thermal load to the lamp refers to thermal load to an arc tube and electrodes of the lamp and can be reduced by a decreased power input.
A variety of techniques have been proposed to achieve light modulation functions or low power lighting of lighting apparatus for high-pressure discharge lamps. These techniques are described below.
The lamp used in a light source of a projector is a high-pressure discharge lamp having a significantly high mercury vapor pressure of, for example, 20 MPa (about 197 atmospheres). The high-pressure discharge lamp has a quartz arc tube and a pair of counter electrodes disposed at a distance of 2 mm or less, and at least 0.15 mg/mm3 of mercury, a rare gas, 10−6 μmol/mm3 to 10−2 μmol/mm3 of halogen that are encapsulated in the arc tube (for example, refer to Japanese Patent Application Publication No. H02-148561). Such a type of discharge lamp and its lighting apparatus is disclosed, for example, in Japanese Patent Application Publication No. 2009-527871. A high-pressure discharge lamp disclosed in Japanese Patent Application Publication No. 2000-131668 has an arc tube with a mercury vapor pressure of 15 MPa to 35 MPa at a stationary power lighting mode and contains 10−6 μmol/mm3 to 10−2 μmol/mm3 of halogen in the arc tube. A pair of electrodes provided in the arc tube each have a protrusion in the central regions of tips of the electrodes to reduce an arc jump phenomenon, which indicates shift of arc generated between the electrodes in the central regions and their vicinities of the tips of the electrodes. A DC voltage is applied from a power supply unit including a DC/DC converter and a DC/AC inverter to the high-pressure discharge lamp to light the lamp.
A modulated power lighting operation of the high-pressure discharge lamp using the power supply unit at a power that is about 70 to 80% of the rated power consumption and at a frequency of pulse-wave AC current that is identical to that in the stationary power lighting causes a flicker phenomenon. This phenomenon is probably due to unstable thermoelectronic emission from low-temperature electrodes.
An attempt to solve such a problem is formation of small projections at tips of the electrodes during lighting of the high-pressure mercury lamp (such projections are not formed during turning-off of the lamp) to stabilize the arc. Such a technology is disclosed, for example, in Japanese Patent Application Publication No. 2006-332015. The technology disclosed in Japanese Patent Application Publication No. 2006-332015 involves intermittent or periodical intercalation of a low frequency to a stationary frequency depending on the lamp voltage or lamp lighting power and changing the frequency or the number of waves at a low-frequency term to maintain projections that function as arc starting points and thus to stabilize the lamp operation.